Evolutionary genetics and biogeography of the marine bryozoan Membranipora membranacea (Cheilostomata): Implications for population history, dispersal routes, and taxonomy

The marine bryozoan Membranipora membranacea (Linnaeus) is widely distributed in temperate oceans of the northern and southern hemisphere and is plastic in taxonomically important characters. In this dissertation, genetic relationships among populations and morphs are determined to better define the species and to elucidate its biogeograhic history. Variation in allozymes and mtDNA are quantified within and among populations of M. membranacea and phylogentic relationships are reconstructed. Within each ocean there is little divergence among samples over broad geographic regions, while large differences exist between populations of different oceans. Genetic diversity and phylogenetic relationships suggest a North Pacific center of origin of M. membranacea. Two hypotheses equally parsimonious with respect to long-distance dispersal events and tropical crossings are discussed for the subsequent migration history. Weighted maximum parsimony suggests dispersal out of the North Pacific southward to Chile in the Late Miocene, eastward to South Africa and Australia, and northward to the North Atlantic in the Early Pliocene. Using genetic divergence as a proxy for reproductive isolation, the relatively low divergence suggests that M. isabelleana and especially M. serrilamella are not separate species but synonyms of M. membranacea. Alternatively, the high divergence of populations in different oceans may indicate a cryptic sibling species complex, a possibility that warrants further investigation. (Abstract shortened by UMI.)...